It has been found that when conducting certain activated biomass treating processes, such as, for example, a waste-water treating process, quantities of phosphorous are taken up by the biomass. It has further been found that during the course of the main processing sequence itself under anaerobic conditions there tends to be a bleed-out of phosphorous from the biomass into the liquor in which the solid biomass is suspended. It has also been found that the employment of aerobic conditions tends to prevent and even reverse this phosphorous bleed-out tendency. In this connection it has been suggested previously in the art to employ an aeration treatment prior to separation by clarification in order to prevent the material in the clarifier from going anaerobic. Discussion of this aspect of the phenomenon can be found, for example, in "A Review of Biological Phosphorous Removal in the Activated Sludge Process" by James L. Barnard, Water SA, Volume 2, Number 3, July 1976, page 136 et seq., particularly at pages 141 and 142, as well as in "Effective Phosphorous Removal from Sewage by Biological Means" by A. R. Mclaren and R. J. Wood in Water SA, Volume 2, Number 1, January 1976, page 47 et seq., particularly at page 50. While the above-mentioned papers are directed to the treatment of the mixed liquor during the processing itself, there is mention in the literature of the effects of reaerating sludge which has gone anaerobic. See for example, "Full Scale Experimentation On The New Johannesburg Extended Aeration Plants " by H. A. Nicholls in Water SA, Volume 1, Number 3, October 1975, page 121 et seq., particularly pages 127 and 128.
The suggestions of the prior art, however, have not directed themselves to the solution of the problem confronting the art today. Specifically, after having substantially completed the treatment of a phosphorous and BOD-containing wastewater, or other influent, in an of activated biomass process, typically after a final oxic or aerobic treatment, the mixed liquor entering the clarifier is composed of a liquid portion of relatively low phosphorous content and a solids or biomass component, relatively rich in phosphorous. This is particularly true when practicing the process of Spector as described in U.S. Pat. No. 4,056,465 to provide a biomass of the type claimed in U.S. Pat. No. 4,162,153. The problem still confronting the art and unresolved by the suggestions of the papers mentioned above is the two-fold problem connected with handling the portion of separated biomass which is not recycled to the process, but rather which is "wasted" and/or recovered as product, so as to insure that the biomass has a high phosphorous content while, at the same time, insuring that the liquor in which it is suspended has a low phosphorous content. It is important that the biomass have a high phosphorous content since such is desirable either as a component of the final biological product or to facilitate final separation of the biomass from the liquor. It is equally important that the liquor have a low phosphorous content since the liquor, once separated, must be disposed of or recovered as a product in which phosphorous is not desired.
Normally, in handling the biomass, particularly when in the form of sludge from a wastewater treating plant, the further treating to which it is subjected is conducted intermittently or in a batch method. Accordingly, therefore, the separated sludge must be stored for some interval of time after removal from the clarifier and before being subjected to the further treatment. The result of this storage is that the biomass goes anaerobic and phosphorous in the form of soluble phosphate bleeds out of the biomass into the liquor in which it is suspended. To overcome these problems and to facilitate the further processing of separated biomass which has gone anaerobic, it is necessary to provide a method for causing rapid reabsorption of phosphorous by the biomass, which method further results in the retention of such reabsorbed phosphorous over a protracted period of time. Unfortunately, the techniques suggested by the prior art do not teach a method for rapidly reabsorbing phosphorous nor do they suggest techniques that will cause extensive reabsorption of phosphorous by the biomass so as to provide a suspending liquor of relatively low phosphorous content. Further, the prior art does not suggest an effective means of retaining the phosphorous in the biomass once it has been reabsorbed. Specifically, the prior art does not solve the problem of rapidly reducing the phosphate content (expressed as mg/l of phosphorous) of the liquor to a level of less than about 5 mg/l and then retaining the phosphate content at this low level for an extended period of time.
This invention is directed to a process which results in a relatively rapid reabsorption of phosphorous into biomass and which also results in the retention of phosphorous in the biomass for an extended period of time.